This invention relates to ultrasonic wave watching devices of moving object detecting type and, more particularly, to a moving object detecting type ultrasonic wave watching device wherein a reflected wave from a moving object in a watching space in which an ultrasonic wave is emitted is received by a receiver and this received signal is treated to detect the presence of a moving object and to operate switch device of an alarm.
The watching device of the kind referred to is to show a watching function to help the security of kept articles by generating a warning by quickly detecting any illegal intruder in such place wherein important articles are retained or kept as, for example, museums, art galleries, show rooms, safe chambers, jewellery stores or the like. The stability of its operation has been influenced by such conditions caused by the setting state or the environment of the setting place of the device as, for example, a reflected wave from a stationary fixed object, a foreign ultrasonic wave noise and the like.
Therefore, the greatest technical object of the device of this kind is to positively obtain only a wave component reflected from a moving object from a received signal including a reflected wave from a moving object, a reflected wave from a stationary or fixed object or objects such as room walls, displayed articles and the like in the watching space, a directly transmitted signal from a transmitter to a receiver and such foreign ultrasonic wave noise as, for example, a sound of a telephone bell, register, typewriter, key bundle or the like, irrespective of any alarm setting state and environment.
There has been already suggested such useful watching device as, for example, of the following type wherein a phenomenon (so-called "Doppler shift") that a frequency of ultransonic wave reflected from a moving object in the watching space shifts from the frequency of emitted ultrasonic wave signal because of the movement of the object is utilized to detect this shifting component to determine the presence of the moving object. In this device, a certain frequency band of such shifts produced, for example, in the case of the movement of a human body is determined in advance by an experiment or calculation and a signal from the moving object is detected by using a filter passing only this frequency band. According to this device, it is possible to positively distinguish or discriminate a reflected wave from a stationary object or a directly received wave from a transmitter to a receiver which does not show the Doppler shift from a reflected wave from a moving object which shows the Doppler shift. However, as the before described foreign ultrasonic wave noise or the frequency component generated by wind which is superposed on the shifting frequency is included in the preset frequency band, the device is still likely to cause a false alarm or indication of the presence of a moving object.
In another watching device suggested to avoid the above described defects, in treating a received signal, its signal component is separated simultaneously into components respectively higher and lower than the frequency f.sub.o of the transmitted signal, they are separately detected and, only in case one of these signals (higher or lower) alone is detected or one signal component is predominantly more than the other, a switch of the device will be actuated.
The treating method of this device is achieved on the basis of, first, the property that the Doppler shift generated by the movement of the object occurs only on one of the upper and lower sides with respect to the transmitted wave frequency to and, second, an assumption that the frequency distribution of the noise occurs generally as substantially uniformly balanced on both upper and lower sides with respect to the transmitted wave frequency f.sub.o.
If this method is used, it is possible to eliminate far more influences of external turbulences than in the case of the first described device but no effect can be expected on noises having a spectrum generated asymmetrically above and below with respect to the transmitted wave frequency and still there has been a defect that a false alarm occurs with a part of foreign noises.